1. Field of the Invention
The present invention relates to a wireless terminal management method in a base station constituting a wireless communication system of a CSMA system together with a wireless terminal group including a plurality of wireless terminals capable of operating intermittently, and more particularly to a wireless terminal management method and a base station, both enabling achievement of lowering of power consumption by enabling the intermittent operations of almost all of the wireless terminals, and further to a wireless terminal management program to be executed by the base station.
2. Description of Related Art
In a wireless LAN system of a carrier sense multiple access (CSMA) system represented by IEEE 802.11, a mobile station should always receive a transmission from a base station, and consequently the power consumption of the mobile station becomes large even if the mobile station does not perform any transmissions and receptions of data. Accordingly, also in IEEE 802.11 Standard, a power save mode is regulated. This is an intermittent operation such that a mobile station is put in a standby state (indicating the same meaning as that of a power save state), in which the mobile station cannot receive any beacon frames, for predetermined beacon frames, and that, after the mobile station is put in a state in which the mobile station can receive beacon frames (active state) owing to a return from the standby state, the mobile station performs transmissions and receptions of accumulated data up to that time, and further that the mobile station is put in the standby state again after that.
In that case, if the return period from the standby state is not shortened as much as possible, the effect of the intermittent operation is worn off. In IEEE 802.11 Standard, a method for making a terminal which has returned from the power save mode perform a transmission and a reception of data preferentially in accordance with a procedure called “PS-POLL” by means of a contention free period (CFP) is prepared.
Hereupon, a network composed of a base station, a normal mode mobile station and power save mode mobile stations is exemplified while a power saving operation according to prior art is described. FIG. 5 shows the operation of an example thereof. As shown in the drawing, beacons are generated and transmitted at a fixed period from a base station AP. Between a beacon and the next beacon, a contention free period CFP, which is defined immediately after the beacon in order that the base station AP can acquire wireless media preferentially and is assigned as a predetermined period, and a contention period CP (a fixed period) for performing a transmission and a reception of data by securing a right of access by the CSMA are provided.
Moreover, marks MT1 to MT4 and vertically long rectangular frame (displayed with oblique lines or without oblique lines), which are shown in the drawing, are described as follows:
MT1: a mobile station which is not put in any power save states;
MT2: a mobile station which is put in a power save state with an interval of four beacons (four beacon periods);
MT3: a mobile station which is put in a power save state with an interval of four beacons or more;
MT4: a mobile station which is put in a power save state with an interval of four beacons or more;
vertically long rectangular frame (displayed with oblique lines): access between the AP and an MT during a CFP for the base station AP to perform polling to the mobile stations MT1 to MT4 in the order of association ID (AID) for performing a transmission and a reception of data; and
vertically long rectangular frame (displayed without any oblique lines): access between the AP and an MT during a CP during which a transmission and a reception of data is actually performed.
Incidentally, the “association” means the communication to be performed for transferring basic information pertaining to the communication such as functions corresponding to the respective base station AP and the mobile stations MT1 to MT4 and respectively necessary transmission bandwidths before actual data communication between each of them.
As shown in FIG. 5, polling in the CFP is performed in the order of the AID. Each of the mobile stations MT2 to MT4, which has returned from its power save state, is put in a beacon frame receivable state (an active state) until the mobile station itself is polled. Then, each of them is again put in its power save state after the end of a transmission and a reception of data during the CP, or is again put in the power save state just behind the polling during the CFP in a case where no transmissions and receptions of data are necessary. Incidentally, in the present example, the order of transmissions and receptions of data in a CP is not always performed in the order of the AID, but the order is determined on all such occasions according to random numbers generated in each of the mobile stations MT1 to MT4.
Generally, in IEEE 802.11 Standard, CFPollable mobile stations (indicating terminals corresponding to polling operations in a CFP, which are options of the standard) are being polled in the order of the AID. Consequently, there is a case where a mobile station which has returned from its power save state is waiting its turn until the mobile station itself is polled. Moreover, when a return timing from a power save mode overlaps in time with a specific beacon frame, the transmission and the reception of the data accumulated during the CFP have not been completed, and the right of access is successively secured in the CSMA to perform a transmission and a reception of the data.
For information, the power save mode in the infrastructure base service set (BSS) mode of the IEEE 802.11 would be described as follows:
1) A mobile station notifies a base station how many times of beacons the mobile station is put in its power save state at the time of an association;
2) The mobile station wakes up in order to be able to receive a prearranged beacon, and confirms whether any data to be received exists in the base station or not;
3) The base station displays that the base station is buffering the data addressed to the mobile station, which is in power saving, in a part of a delivery traffic indication map (DTIM: the information concerning the beacon frame at which a multicast frame transmission from an access point is performed);
4) The mobile station, which has returned, confirms a traffic indication map (TIM). Then, when any data exists, the mobile station receives the data in accordance with the PS-POLL;
5) The mobile station confirms the DTIM. Then, the mobile station also wakes up by the beacon by which a multicast by the base station is performed; and
6) The mobile station transmits a frame of a power saving start to the base station, and returns to its standby state at the point of time when the mobile station receives an acknowledgement (Ack) from the base station.
Incidentally, Patent Document 1 discloses a method for performing the transmission of data addressed to a mobile station operating intermittently at the timing when the mobile station returns preferentially to the transmissions and the receptions of data addressed to the other mobile stations operating normally, though the method has no compatibility with the system of IEEE 802.11 Standard, which has begun to be spread as a system of a wireless LAN. Moreover, Patent Document 1 also discloses a method for making an interval of beacons variable according to the data amount of a mobile station operating intermittently, and a method for changing the timing for returning to a standby state. On the other hand, Patent Document 2 also discloses an intermittent operation as a technique for differentiating a check period of base station's checking.    [Patent Document 1] Japanese Patent Application Publication (KOKAI) No. Hei 9-162798    [Patent Document 2] Japanese Patent Application Publication (KOKAI) No. 2003-110582